Early defibrillation is critically important in the management of out-of-hospital cardiac arrest due to ventricular fibrillation. In many urban areas, however, paramedics may be unable to respond to the scene within a time frame optimal for successful resuscitation. Rapid defibrillation by first-responders using an automatic external defibrillator (AED) has been proposed to increase the rate of resuscitation, but evidence supporting the value of this intervention in urban areas is scant. This research proposal outlines population based, controlled clinical trial of first-responder defibrillation for management of out-of-hospital cardiac arrest in an urban EMS system already served by paramedics. Thirty engine companies of the Memphis, Tennessee Fire Department will be selected to participate in the trial. All will be trained to operate an AED and all will receive refresher training in cardiopulmonary resuscitation (CPR). Fifteen engine companies will be randomly selected to employ an AED during the initial 75 days of the trial. The remaining 15 companies will provide cardiopulmonary resuscitation (CPR) until paramedic arrival and will serve as controls. Once a paramedic unit arrives, all patients will receive standard prehospital advanced cardiac life support and will be transported to the nearest hospital emergency department. Every 75 days thereafter, participating engine companies will exchange the device in order to control for training effects and possible selection bias. Over a 24 month study period, the rate of successful resuscitation, survival to hospital discharge, and functional status at discharge for patients receiving first responder defibrillation will be compared to that obtained by patients receiving CPR only prior to paramedic arrival. A significant improvement in outcomes due to first responder defibrillation will validate this concept in urban EMS systems already served by paramedics and support adoption of this program on an ongoing basis by the city of Memphis.